You can dismiss every major historical event with this kind of reasoning. Everything is, at some level, based on luck and timing.
Bottom line, unlike the voyages of Polynesians and Vikings, the discovery of the Americas was an incredibly important historical event and is a foundation of the current global order.
>The notion of the progress and advancement inherent to Europeans as "discovers" is propaganda.
No, it isn't. Europeans were, objectively, far more technologically advanced than natives of the Americas.
You keep getting hung up on the word 'discover'. Why? From the perspective of Europeans, it was a discovery. They found a landmass that they didn't know existed. We just recently 'discovered' gravitational waves, even though those specific ones were/are emitted for eons.
>We celebrate it because they killed most of the people
Don't be dense. That's not why we celebrate it - and you know it.
The Europeans didn't kill most of the inhabitants. The vast vast majority of natives died due to Old-World diseases, not through war or murder. Second, Europeans were not a monolithic group and colonization was not a singular event. It spanned centuries, involved many groups of people and took place on an immense landmass (the Americas). It also occurred during a transition from late medieval era to modern era. So your simplistic view of "(Europeans) killed most of the people who were there first and got claim it all for themselves" is simplistic and stupid.
There is evidence that Polynesians made it to the "new world" long before Europeans (http://www.mnn.com/earth-matters/wilderness-resources/storie...). And even in the case of Europenas, they were able to do it with the technology invented in 10th/11th centuries e.g. Leif Erickson's voyage. It was just that the political and economic climes were not optimal for them to sail West until centuries later.
The notion of the progress and advancement inherent to Europeans as "discovers" is propaganda. We celebrate it because they killed most of the people who were there first and got claim it all for themselves.